I think the problem is trying to compare different models trained don the same dataset.
1) If I compare for example gam (from gam package) with and without intercept, is that a valid comparison? For example: model with intercept has explained dev 24%, with AIC -2217146, model without intercept has explained dev 85.5% with AIC 217488.1 The results sound incredibly strange, but there is actually no difference in the model but the removal of the intercept .... :(. So which model is "better" at fitting the data???? 2) If I compare for example gam from gam package with let's say gam from mgcv (using tpsp), then I get two completely analogous AIC, but are they comparable? gam from mgcv package: -2195000 gam from gam package: -2217000 3) I would like to compare those AIC to the AIC obtained by running BRT on the same dataset. I was thinking of simply recalculating manually the AIC using the formula: AIC=2K+N*log(rss/N) where K is the number of parameters of the regression (i.e. the coefficient that are not zero, I would think) and N is the number of samples. What do you think? Would that be reasonable? Regards On Thursday 10 September 2009 16:39:32 Ben Bolker wrote: > If all the models are fitted to the same data set, using the same > modeling tools (you have to be careful e.g. comparing lmer models to > glm models, because they use different additive constants), and > everything seems to make sense (!!!), then yes. I would be a little > surprised, and think that something was wrong, if you have some AIC > values that are on the order of -20,000 (as below) and others that are > +20,000 ... > > Ben Bolker > > Corrado wrote: > > My worry is: can I compare negative AIC with positive AIC? does the > > comparison still hold? > > > > On Thursday 10 September 2009 15:57:01 Ben Bolker wrote: > >> Corrado-5 wrote: > >>> Dear R list, > >>> > >>> I just obtained a negative AIC for two models (-221.7E+4 > >>> and -230.2E+4). Is that normal? > >> > >> It's not necessarily wrong. See <http://emdbolker.wikidot.com/faq> -- Corrado Topi Global Climate Change & Biodiversity Indicators Area 18,Department of Biology University of York, York, YO10 5YW, UK Phone: + 44 (0) 1904 328645, E-mail: ct...@york.ac.uk ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.